Catalytic oxidation of nox/sox in flue gases with atmospheric oxygen as the oxidation reagent

ABSTRACT

The present invention solves the existing problem of using very expensive oxidation reagents, such as H2O2 and ozone, in removal of NOx and SOx from flue gases, by performing simultaneous oxidation of NOx and SOx with atmospheric oxygen in a combined system for catalytic oxidation and wet-scrubbing of both NOx and SOx from a flue gas and manufacturing fertilisers. Two major configurations of the oxidation system are disclosed in the present invention. The first configuration operates on oxygen-enriched air to increase efficiency of the oxidation reaction and requires an additional oxygen concentrator unit. The second configuration operates on atmospheric air at ambient conditions and requires an additional catalyst activation unit. In the second configuration, the efficient oxidation process is carried out at low temperatures of about 30-90° C. in the presence of recovered and re-activated catalyst. This temperature is a result of the exothermic character of the reaction, and therefore, no heating is required in the process.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. application Ser. No.16/628,028 filed on Jan. 1, 2020, which is the National Phase of PCTPatent Application No. PCT/IL2018/050793 having International filingdate of Jul. 18, 2018, which claims the benefit of priority of U.S.Provisional Application No. 62/534,805 filed on Jul. 20, 2017. Thecontents of the above applications are all incorporated by reference asif fully set forth herein in their entirety.

TECHNICAL FIELD

The present invention relates to a method and system for combinedcatalytic oxidation of nitrogen oxides (NO_(x)) and sulphur oxides(SO_(x)) in flue gases. In particular, the present invention relates touse of atmospheric oxygen as the oxidation reagent in the combinedcatalytic oxidation process of nitrogen oxides (NO_(x)) and sulphuroxides (SO_(x)) in flue gases.

BACKGROUND

Nitrogen oxides (NO_(x)) and sulphur oxides (SO_(x)) are air pollutantsemitted in large quantities from nitrogen- and sulphur-contaminatedfossil fuel industrial plants, such as power plants. Two of the mostcommon nitrogen oxides are nitric oxide (NO) and nitrogen dioxide (NO₂).Nitric oxide (NO) is a colourless to brown gas at room temperature witha sharp and sweet smell. Nitrogen dioxide (NO₂) is a reddish-brown gasat temperatures above 20° C. and becomes colourless to brown liquid,with a strong and harsh odour on cooling. It is highly reactive andexists in equilibrium with the colourless gas dinitrogen tetroxide(N₂O₄):2NO₂⇄N₂O₄. Unlike NO₂, N₂O₄ is diamagnetic since it has nounpaired electrons. N₂O₄ can be crystallised as a white solid having amelting point −11.2° C. The liquid N₂O₄ is also colourless but canappear as a brownish yellow liquid due to the presence of NO₂ accordingto the above equilibrium. The equilibrium is exothermic andcharacterised by ΔH=−57.23 kJ/mol. Thus, higher temperatures push theequilibrium towards NO₂, while at lower temperatures, dinitrogentetroxide (N₂O₄) predominates. Inevitably, some N₂O₄ is a component ofNO₂-containing smog. Nitrous oxide (N₂O) is a well-known greenhouse gasthat contributes to climate change. Sulphur dioxide (SO₂) is thepredominant form of the sulphur oxides found in the lower atmosphere. Itis a colourless gas that can be detected by taste and smell in the rangeof 1,000 to 3,000 micrograms per cubic meter (μg/m³).

NO_(x) and SO_(x) are released into the air not only from power plantsbut from any motor vehicle exhaust or the burning of coal, oil, dieselfuel, and natural gas, especially from electric power plants. They arealso released during industrial processes such as welding,electroplating, engraving, and dynamite blasting. They may also beproduced by cigarette smoking.

Nitrogen oxides, when combined with volatile organic compounds, formground-level ozone, or smog. NO_(x) and SO_(x) react with oxygen andundergo reactions with water vapours in the atmosphere to yield acidrains. These oxides are the most common pollutants found in the airaround the world. Exposure to high industrial levels of NO_(x) andSO_(x) can cause collapse, rapid burning and swelling of tissues in thethroat and upper respiratory tract, difficult breathing, throat spasms,and fluid build-up in the lungs. It can interfere with the blood'sability to carry oxygen through the body, causing headache, fatigue,dizziness and eventually death. Therefore, in accordance with stringentenvironmental restrictions regarding pollutant emissions the removal ofthese pollutants from industrial gas streams is very important andfollows.

Currently, in most industrial processes, NO_(x) and SO_(x) are treatedseparately. Emissions of SO_(x) are being reduced significantly andcompletely removed from flue gases by wet scrubbing technology using aslurry of alkaline sorbent, usually limestone or lime, or seawater toscrub gases. Sulphur dioxide is an acid gas, and, therefore, the typicalsorbent slurries or other materials used to remove the SO₂ from the fluegases are alkaline. In some designs, the product of the SO₂ wetscrubbing, calcium sulphite (CaSO₃), is further oxidised to producemarketable gypsum (CaSO₄.2H2O). This technique is also known as forcedoxidation, flue gas desulfurization (FGD) or fluidised gypsumdesulfurization. It is the most effective technology for SO_(x) removal.

Although the flue gas desulfurization process achieves relatively highSO_(x) removal efficiency, it is not effective in NO_(x) removal. Thisis because nitric oxide (NO) gas, which comprises more than 90% ofNO_(x) in the flue gas, is quite insoluble in water. The oxidation ofnitrogen to its higher valence states yields NO_(x) soluble in water.When this is carried out, a gas absorber becomes effective. In general,an oxidiser must be added to the scrubbing system in order to convertinsoluble NO gas (5.6 mg per 100 ml of water at room temperature) tosoluble NO₂ as a prerequisite step.

Thus, absorption of NO_(x) gases is probably the most complex whencompared with other absorption operations because of nitric oxide lowsolubility. Therefore, the control of NO_(x) is mostly achieved by usingchemical reduction technique, for example, selective catalytic reduction(SCR). The yields of nitrogen oxides reduction to nitrogen in the SCRare typically high, but this technique is extremely expensive.

Kasper et al (1996) in “Control of Nitrogen Oxide Emissions by HydrogenPeroxide-Enhanced Gas-Phase Oxidation Of Nitric Oxide”, Journal of theAir and Waste Management Association 46(2), pages 127-133, describedthat the removal of NO_(x) in wet scrubbers may be greatly enhanced bygas-phase oxidation of water-insoluble NO gas to water-soluble NO₂,HNO₂, and HNO₃ (the acid gases are much more soluble in water thannitric oxide). The gas-phase oxidation may be accomplished by injectingliquid hydrogen peroxide into the flue gas, so that H₂O₂ vaporises anddissociates into hydroxyl radicals. The gas-phase oxidation may beaccomplished by injecting liquid hydrogen peroxide (H₂O₂) into the fluegas at temperatures higher than 300° C., so that H₂O₂ vaporises anddissociates into hydroxyl radicals. The oxidised NO_(x) species may thenbe easily removed by caustic water scrubbing.

Oxidants that have been injected into the gas flow are ozone, ionisedoxygen or hydrogen peroxide. Non-thermal plasma generates oxygen ionswithin the air flow to achieve this. Other oxidants have to be injectedand mixed in the flow. The kinetic problem of fast oxidation of nitrogenand sulphur oxides, NO to NO₂ and SO₂ to SO₃, during the short residencetime in the exhaust stack has been taken care by using very strongoxidants like ozone or hydrogen peroxide. Stamate et al (2013) in“Investigation of NO _(x) Reduction by Low Temperature Oxidation UsingOzone Produced by Dielectric Barrier Discharge”, Japanese Journal ofApplied Physics 52(5S2), 05EE03, suggested that in order to enhance thewet scrubber operation, ozone may be used for NO_(x) gases oxidation.Stamate and Stalewski (2012) in “NO _(x) reduction by ozone injectionand direct plasma treatment”, Proceedings of ESCAMPIG XXI, Viana doCastelo, Portugal, Jul. 10-14, 2012, compared the NO_(x) reduction byozone injection with direct plasma treatment. Hutson et al. (2008) in“Simultaneous Removal of SO ₂ , NO _(x) , and Hg from Coal Flue GasUsing a NaClO ₂-Enhanced Wet Scrubber”, Industrial and EngineeringChemistry Research (I&EC) 47(16), pages 5825-5831, taught using sodiumchlorite NaClO₂ as an oxidiser for removal of NO_(x) gases.

Thus, the aforementioned oxidation techniques intentionally raise thevalence of nitrogen in a nitrogen oxide to allow water to absorb theoxidised nitrogen oxide. This is accomplished either by using acatalyst, injecting hydrogen peroxide, creating ozone within the gasflow, or injecting ozone into the gas flow. Non-thermal plasma, whenused without a reducing agent, can be used to oxidise NO_(x) as well. Awet scrubber must be added to the process in order to absorb N₂O₅emissions into the atmosphere. Any resultant nitric acid may then beneutralised by a scrubber liquid and then sold (usually as a calcium orammonia salt to produce fertilisers). Alternatively, it may be collectedas nitric acid for sales.

However, the above processes using hydrogen peroxide (H₂O₂) and ozone(O₃) require expensive and corrosion-resistant systems to be installedin the wet scrubber units, which significantly increase the productioncosts of both hydrogen peroxide and ozone (which are expensive reagentsthemselves) used for oxidation of NO_(x) and SO_(x) gases. This, inturn, requires the introduction of the expensive production units whichalso use high voltage and high safety standards. Therefore, theoxidation methods mentioned above for wet scrubbing technology areextremely expensive. In addition, both H₂O₂ and O₃ are reactive andcorrosive, which creates several maintenance problems. Therefore,despite recent developments in this field, there still remains a needfor a process, which would be economical (cheap), safe and easilyup-scaled for industrial needs, and which would effectively removesimultaneously both NO_(x) and SO_(x) from the flue gases in the widerange of industrial applications.

SUMMARY

The aforementioned problems in removal of both NO_(x) and SO_(x)simultaneously from the flue gases using expensive oxidation reagents,such as H₂O₂ and ozone, may be solved by using atmospheric oxygeninstead as an oxidation reagent. In one embodiment, a combined systemfor catalytic oxidation and wet-scrubbing of simultaneously bothnitrogen oxides (NO_(x)) and sulphur oxides (SO_(x)) from a flue gas,and for manufacturing fertilisers comprising:

a) An oxidation reactor (1) filled with an oxidation catalyst or with anadsorbing dispersion containing said oxidation catalyst and designed:

-   -   to receive either (i) a mixture of oxygen-enriched air streamed        from an oxygen concentrator and a flue gas containing NO_(x) and        SO_(x), or (ii) a mixture of ambient air and the flue gas        containing NO_(x) and SO_(x);    -   to adsorb said gases on the particles of the oxidation catalyst;    -   to carry out catalytic oxidation of said NO_(x) and SO_(x) to        yield oxidised NO_(x) and SO_(x), and    -   to perform wet-scrubbing of said oxidised NO_(x) and SO_(x),        thereby yielding nitric and sulphuric acids;        b) A vessel containing gas or liquid ammonia, connected to the        oxidation reactor (1) or to a separating and reactor-controlling        unit (2), said vessel having an inlet configured to stream said        ammonia into the oxidation reactor (1) or into the separating        and reactor-controlling unit (2) for reacting with the obtained        nitric and sulphuric acids and to yield ammonium nitrate and        ammonium sulphate fertilisers, thereby wet-scrubbing NO_(x) and        SO_(x) from the flue gas and producing said fertilisers; and        c) The separating and reactor-controlling unit (2) connected to        said oxidation reactor (1) and designed to separate the obtained        products (fertilisers) and liquids, and to control said        catalytic oxidation reaction and wet-scrubbing of the gases in        the reactor (1),        wherein if the oxidation reactor (1) is configured to receive        the mixture of ambient air and the flue gas containing NO_(x)        and SO_(x), said system further comprises an activation chamber        (8) for activating the oxidation catalyst, said activation        chamber (8) contains an activation reagent and is in fluid        communication with the separating and reactor-controlling unit        (2), from which it receives the deactivated oxidation catalyst,        and with the oxidation reactor (1), to which it feeds the        oxidation catalyst after its activation.

In some embodiments, when the oxidation reactor (1) is configured toreceive a mixture of oxygen-enriched air and a flue gas (i.e. the systemoperates on the oxygen-enriched air), the system further comprises theoxygen concentrator designed to concentrate oxygen from ambient air andgenerate an air stream enriched with oxygen. The oxygen concentrator(not shown in the figures) is in fluid communication with the oxidationreactor (1). The mixture of the oxygen-enriched air and flue gasstreamed into the oxidation reactor (1) consists of about 50-80% of theoxygen-enriched air and 20-50% of the flue gas.

The activation reagent in the activation chamber (8), capable ofactivating the deactivated oxidation catalyst at temperatures about30-90° C., is selected from any suitable strong oxidiser, non-limitingexamples of which are hydrogen peroxide (H₂O₂), hydroperoxides, such astert-butyl hydroperoxide, benzoyl peroxide and the like.

The oxidation reactor (1) may be dry and packed with inert solids, suchas ceramic beads, promoting a better contact between said oxygen streamand said flue gas stream, or wet containing a liquid circulating inside.In a specific embodiment, the oxidation reactor (1) is selected from abubble column, packed bed and spray tower equipped with spray means.These spray means can spray into the spray tower either: (i) water ormother liquor on the dry oxidation catalyst particles, thereby formingfloating drops of the adsorbing dispersion directly inside the spraytower, or (ii) the adsorbing dispersion prepared in advance andcontaining the oxidation catalyst. The spray tower may be a wet scrubbercomprising an empty cylindrical vessel made of steel or plastic, andinlets for gas streams. The spray means may comprise one or more spraynozzles arrayed within the spray tower along the flue gas flow path andconfigured to spray said water, mother liquor or adsorbing dispersioninto the vessel. These spray nozzles are equipped with a demister (3)for mist removal.

In a further embodiment, the oxidation catalyst may comprise the mixtureof an aqueous solution of a metal salt precursor with silica gelparticles and used for catalysing the oxidation reaction of NO_(x) andSO_(x) in the flue gas. The metal salt precursor is a water-solubleinorganic salt of a transition metal selected from cobalt (Co), nickel(Ni), manganese (Mn), iron (Fe), copper (Cu) and chromium (Cr). In aspecific embodiment, the metal salt precursor is cobalt chloride(CoCl₂). The oxidation catalyst may also comprise an aqueous suspensionof cobalt oxide/hydroxide particles supported on silica gel particles.

In yet further embodiment, the separating and reactor-controlling unit(2) comprises at least one of the following processing units: a phaseseparator, crossflow separator, mixer-settler, decanter, tricanter orfilter. It may further comprise sensors for measuring and controllingtemperature and pH of the processed liquids. When the adsorbingdispersion is an oil-water emulsion, the separator and reactor controlunit comprise an oil-water phase separator configured to receive saidoil-water emulsion from the adsorption and oxidation reactor and toseparate the oil (organic phase) from water (aqueous phase). The organicphase of the oil-water emulsion of this embodiment may contain elementalsulphur in saturated heavy mineral oil. Sulphur is capable of catalysingthe reaction of the nitric and sulphuric acids with ammonia to yield theammonium nitrate and ammonium sulphate as fertiliser products dissolvedin the aqueous phase. The organic phase may further comprise activators,such as dichlorobenzene, Disperbyk®-108, decabromodiphenyl ether ordiphenyl ether, added to the sulphur oil solution to increase thesolubility of the elemental sulphur.

In another embodiment, the system of the present invention furthercomprises a crystallisation vessel connected to the processing unit andconfigured to receive from the processing unit an aqueous solutioncontaining the dissolved ammonium nitrate and ammonium sulphateproducts. The crystallisation vessel is capable of crystallising andprecipitating the ammonium nitrate and ammonium sulphate from theaqueous solution.

In still another embodiment, the system of the present invention furthercomprises a separate oxidation chamber (4). This oxidation chamber (4)is either connected to the oxygen concentrator and configured to receivethe mixture of the oxygen-enriched air stream and the stream of the fluegas containing NO_(x) and SO_(x), or connected to a separate vessel (notshown here), which contains the activation reagent, and configured toreceive the stream of the flue gas with ambient air and the activationreagent from said separate vessel. This oxidation chamber (4) isinstalled in fluid communication with the oxidation reactor (1), ispre-filled with the oxidation catalyst and is capable of carrying thecatalytic oxidation of NO_(x) and SO_(x) in the flue gas. It may be dryand packed with inert solids, such as ceramic beads, promoting a bettercontact between said air stream and said flue gas stream, or wetcontaining a liquid circulating inside.

In a further embodiment, a method for catalytic oxidation and removal ofnitrogen oxides (NO_(x)) and sulphur oxides (SO_(x)) simultaneously froma flue gas containing said oxides, comprising:

I. Catalytic oxidation of the NO_(x) and SO_(x) contained in the fluegas with oxygen, wherein said catalytic oxidation yields the oxidisedNO_(x) and SO_(x), said catalytic oxidation is carried out in theoxidation reactor (1) of the system of claim 1; andII. Wet-scrubbing of the oxidised NO_(x) and SO_(x) with an adsorbingdispersion comprising a solid oxidation catalyst suspended in water, anoxidation catalyst soluble in organic solvent and emulsified in water,or combination thereof, thereby removing the NO_(x) and SO_(x) from theflue gas and yielding nitric and sulphuric acids dissolved in water.

The above process further comprises the step of contacting the oxidisedNO_(x) and SO_(x) dissolved in a liquid phase of said adsorbingdispersion, with ammonia to produce ammonium nitrate NH₄NO₃ and ammoniumsulphate (NH₄)₂SO₄ used as fertilisers. The obtained product is furthersubjected to separation and crystallisation. In yet further embodiment,the above process comprises the steps of crystallisation, precipitationand collection of the NH₄NO₃ and (NH₄)₂SO₄ products from the aqueoussolution, and recycling of water from mother liquor left afterprecipitation of the ammonium nitrate and ammonium sulphate products.

In a specific embodiment, the above process is carried out in thetemperature range of about 30-90° C. and pH 4-7. This pH is maintainedwith ammonium hydroxide injection in order to keep the reaction goingand not to create the alkaline solution, from which ammonia gas (NH₃)may evolve.

No heating is required in the process, because the process isexothermic. The observed elevated temperature of about 30° C. to 90° C.originates from the thermodynamic equilibrium, and therefore, noadditional heat is applied.

Various embodiments of the invention may allow various benefits, and maybe used in conjunction with various applications. The details of one ormore embodiments are set forth in the accompanying figures and thedescription below. Other features, objects and advantages of thedescribed techniques will be apparent from the description and drawingsand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description taken in conjunction with theappended figures. Various exemplary embodiments are well illustrated inthe accompanying figures with the intent that these examples not berestrictive. Of the accompanying figures:

FIG. 1a shows the operational diagram of the system of the presentinvention for combined catalytic oxidation and wet-scrubbing of nitrogenoxides (NO_(x)) and sulphur oxides (SO_(x)) from flue gases, where thesystem operates on oxygen-enriched air streamed from an oxygenconcentrator.

FIG. 1b shows the operational diagram of the system of the presentinvention for combined catalytic oxidation and wet-scrubbing of nitrogenoxides (NO_(x)) and sulphur oxides (SO_(x)) from flue gases, where thesystem operates on ambient (atmospheric) air.

FIG. 2a schematically shows the one-stage system of the presentembodiment, wherein the adsorption and oxidation reactor (1) is a spraytower, where the system operates on oxygen-enriched air streamed from anoxygen concentrator.

FIG. 2b schematically shows the one-stage system of the presentembodiment, wherein the adsorption and oxidation reactor (1) is a spraytower, where the system operates on ambient (atmospheric) air.

FIGS. 3a-3h show the scanning electron microscope (SEM) images of theobtained silica particles coated with cobalt hydrous oxide. According tothe EDS analysis of cobalt, the weight of cobalt is 17.6% (σ=0.8) andthe weight of oxygen is 82.4% (σ=0.8) in the catalyst.

FIG. 4a schematically shows the separator and reactor control unit (2)for the system shown in FIGS. 1a and 2a of the present invention, wherethe system operates on oxygen-enriched air streamed from an oxygenconcentrator.

FIG. 4b schematically shows the separator and reactor control unit (2)for the system where the adsorbing dispersion contains the oil-wateremulsion of the catalyst, and where the system operates onoxygen-enriched air streamed from an oxygen concentrator.

FIG. 4c schematically shows the separator and reactor control unit (2)for the system shown in FIGS. 1b and 2b of the present invention, wherethe system operates on ambient (atmospheric) air.

FIG. 4d schematically shows the separator and reactor control unit (2)for the system where the adsorbing dispersion contains the oil-wateremulsion of the catalyst, and where the system operates on ambient(atmospheric) air.

FIG. 5a shows the operational diagram of the industrial two-stage systemof the present invention with the separate oxidation chamber (4)containing the oxidation catalyst, and where the system operates onoxygen-enriched air streamed from an oxygen concentrator.

FIG. 5b shows the operational diagram of the industrial two-stage systemof the present invention with the separate oxidation chamber (4)containing the oxidation catalyst, and where the system operates onambient (atmospheric) air.

FIG. 6a schematically shows the two-stage system of the presentinvention, wherein the wet scrubber (1) is a spray tower that is capableof spraying the adsorbing dispersion, and where the system operates onoxygen-enriched air streamed from an oxygen concentrator.

FIG. 6b schematically shows the two-stage system of the presentinvention, wherein the wet scrubber (1) is a spray tower that is capableof spraying the adsorbing dispersion, and where the system operates onambient (atmospheric) air.

FIGS. 7 and 8 schematically show the corresponding one- and two-stagelaboratory systems of the present invention shown in FIGS. 2a and 6a ,respectively.

FIG. 9 schematically shows the operational diagram of the one-stagelaboratory system of the present invention fed with synthetic gases.

FIG. 10 schematically shows the operational diagram of the one-stagelaboratory system of the present invention fed with the fuel gases froma diesel engine.

DETAILED DESCRIPTION

In the following description, various aspects of the invention will bedescribed. For purposes of explanation, specific aspects and details areset forth in order to provide a thorough understanding of the invention.However, it will also be apparent to one skilled in the art that theinvention may be practiced without the specific details presentedherein. Furthermore, well-known features may be omitted or simplified inorder not to obscure the invention.

The terminology used herein is for describing particular embodimentsonly and is not intended to be limiting of the invention. The terms“comprising” and “comprises”, used in the claims, should not beinterpreted as being restricted to the means listed thereafter; they donot exclude other elements or steps. They need to be interpreted asspecifying the presence of the stated features, integers, steps and/orcomponents as referred to, but does not preclude the presence and/oraddition of one or more other features, integers, steps or components,or groups thereof. Thus, the scope of the expression “a devicecomprising x and z” should not be limited to devices consisting only ofcomponents x and z. Also, the scope of the expression “a methodcomprising the steps x and z” should not be limited to methodsconsisting only of these steps.

Unless specifically stated, as used herein, the term “about” isunderstood as within a range of normal tolerance in the art, for examplewithin two standard deviations of the mean. In one embodiment, the term“about” means within 10% of the reported numerical value of the numberwith which it is being used, preferably within 5% of the reportednumerical value. For example, the term “about” can be immediatelyunderstood as within 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%,0.1%, 0.05%, or 0.01% of the stated value. In other embodiments, theterm “about” can mean a higher tolerance of variation depending on forinstance the experimental technique used. Said variations of a specifiedvalue are understood by the skilled person and are within the context ofthe present invention. As an illustration, a numerical range of “about 1to about 5” should be interpreted to include not only the explicitlyrecited values of about 1 to about 5, but also include individual valuesand sub-ranges within the indicated range. Thus, included in thisnumerical range are individual values such as 2, 3, and 4 andsub-ranges, for example from 1-3, from 2-4, and from 3-5, as well as 1,2, 3, 4, 5, or 6, individually. This same principle applies to rangesreciting only one numerical value as a minimum or a maximum. Unlessotherwise clear from context, all numerical values provided herein aremodified by the term “about”. Other similar terms, such as“substantially”, “generally”, “up to” and the like are to be construedas modifying a term or value such that it is not an absolute. Such termswill be defined by the circumstances and the terms that they modify asthose terms are understood by those of skilled in the art. Thisincludes, at very least, the degree of expected experimental error,technical error and instrumental error for a given experiment, techniqueor an instrument used to measure a value.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items. Unless otherwise defined,all terms (including technical and scientific terms) used herein havethe same meaning as commonly understood by one of ordinary skill in theart to which this invention belongs. It will be further understood thatterms, such as those defined in commonly used dictionaries, should beinterpreted as having a meaning that is consistent with their meaning inthe context of the specification and relevant art and should not beinterpreted in an idealized or overly formal sense unless expressly sodefined herein. Well-known functions or constructions may not bedescribed in detail for brevity and/or clarity.

It will be understood that when an element is referred to as being “on”,“attached to”, “connected to”, “coupled with”, “contacting”, etc.,another element, it can be directly on, attached to, connected to,coupled with or contacting the other element or intervening elements mayalso be present. In contrast, when an element is referred to as being,for example, “directly on”, “directly attached to”, “directly connectedto”, “directly coupled” with or “directly contacting” another element,there are no intervening elements present. It will also be appreciatedby those of skill in the art that references to a structure or featurethat is disposed “adjacent” another feature may have portions thatoverlap or underlie the adjacent feature.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the specification andrelevant art and should not be interpreted in an idealised or overlyformal sense unless expressly so defined herein. Well-known functions orconstructions may not be described in detail for brevity and/or clarity.

The deficiencies of the prior art as discussed above are alleviated bythe systems and processes described in the present application, whereinnitrogen oxides (NO_(x)) and sulphur oxides (SO_(x)) of the emitted fluegases are simultaneously oxidised by atmospheric oxygen in the presenceof a catalyst followed by wet scrubbing in the presence of ammonia toyield the corresponding ammonium salts used as fertilisers foragriculture.

As mentioned above, the main problem associated with the existingmethods for removal of NO_(x) and SO_(x) is the very low solubility ofthe nitric oxide gas in water. The oxidation of nitrogen to its highervalence states yields NO_(x) soluble in water. Therefore, the removal ofNO_(x) in wet scrubbers may be greatly enhanced by gas-phase oxidationof water-insoluble NO gas to water-soluble NO₂, HNO₂, and HNO₃ (the acidgases are much more soluble in water than nitric oxide). The gas-phaseoxidation may be accomplished by injecting liquid hydrogen peroxide(H₂O₂) into the flue gas, so that hydrogen peroxide vaporises anddissociates into hydroxyl radicals. Ozone (O₃) can also be used for theoxidation purposes. The oxidised NO_(x) species may then be easilyremoved by caustic water scrubbing.

However, the use of either O₃ or H₂O₂ as oxidising reagents for NO_(x)and SO_(x) creates a series of safety and maintenance problems, becausethese oxidising reagents are relatively expensive, very reactive andcorrosive. As a result, their use in wet scrubbing of NO_(x) and SO_(x)significantly increases the operational and maintaining costs of theprocess and system.

Instead of using O₃ or H₂O₂ for oxidation, the present inventorssuggested using atmospheric oxygen (O₂), which is abundance in the air,considerably reduces both the operational and maintenance cost of thewet scrubber and produces safer and less hazardous working environment.The use of the atmospheric O₂ obviates the need for using the cumbersomeand expensive H₂O₂/O₃ oxidation systems and allows combining thecatalytic oxidation and wet-scrubbing of the flue gas in one-stepprocess.

The present invention describes a combined system for catalyticoxidation and wet-scrubbing of simultaneously both nitrogen oxides(NO_(x)) and sulphur oxides (SO_(x)) from a flue gas, and formanufacturing fertilisers comprising:

a) An oxidation reactor (1) filled with an oxidation catalyst or with anadsorbing dispersion containing said oxidation catalyst and designed:

-   -   to receive either (i) a mixture of oxygen-enriched air streamed        from an oxygen concentrator and a flue gas containing NO_(x) and        SO_(x), or (ii) a mixture of ambient air and the flue gas        containing NO_(x) and SO_(x);    -   to adsorb said gases on the particles of the oxidation catalyst;    -   to carry out catalytic oxidation of said NO_(x) and SO_(x) to        yield oxidised NO_(x) and SO_(x), and    -   to perform wet-scrubbing of said oxidised NO_(x) and SO_(x),        thereby yielding nitric and sulphuric acids;        b) A vessel containing gas or liquid ammonia, connected to the        oxidation reactor (1) or to a separating and reactor-controlling        unit (2), said vessel having an inlet configured to stream said        ammonia into the oxidation reactor (1) or into the separating        and reactor-controlling unit (2) for reacting with the obtained        nitric and sulphuric acids and to yield ammonium nitrate and        ammonium sulphate fertilisers, thereby wet-scrubbing NO_(x) and        SO_(x) from the flue gas and producing said fertilisers; and        c) The separating and reactor-controlling unit (2) connected to        said oxidation reactor (1) and designed to separate the obtained        products (fertilisers) and liquids, and to control said        catalytic oxidation reaction and wet-scrubbing of the gases in        the reactor (1),

wherein if the oxidation reactor (1) is configured to receive themixture of ambient air and the flue gas containing NO_(x) and SO_(x),said system further comprises an activation chamber (8) for activatingthe oxidation catalyst, said activation chamber (8) contains anactivation reagent and is in fluid communication with the separating andreactor-controlling unit (2), from which it receives the deactivatedoxidation catalyst, and with the oxidation reactor (1), to which itfeeds the oxidation catalyst after its activation.

The present invention discloses two configurations of the oxidationsystem. The first configuration operates on oxygen-enriched air toincrease efficiency of the oxidation reaction and requires an additionaloxygen concentrator unit. The second configuration operates onatmospheric air at ambient conditions and requires an additionalcatalyst activation unit. In the second configuration, the efficientoxidation process is carried out at low temperatures of about 30-90° C.in the presence of recovered and re-activated catalyst. This temperatureis a result of the exothermic character of the reaction, and therefore,no heating is required in the process.

Thus, in some embodiments, when the oxidation reactor (1) is configuredto receive a mixture of oxygen-enriched air and a flue gas (i.e. thesystem operates on the oxygen-enriched air), the system furthercomprises the oxygen concentrator designed to concentrate oxygen fromambient air and generate an air stream enriched with oxygen. The oxygenconcentrator (not shown in the figures) is in fluid communication withthe oxidation reactor (1). The mixture of the oxygen-enriched air andflue gas streamed into the oxidation reactor (1) consists of about50-80% of the oxygen-enriched air and 20-50% of the flue gas.

Reference is now made to FIG. 1a showing the operational diagram of thesystem of the present invention for combined catalytic oxidation andwet-scrubbing of nitrogen oxides (NO_(x)) and sulphur oxides (SO_(x))from flue gases and manufacturing fertilisers from them, where thesystem operates on oxygen-enriched air streamed from an oxygenconcentrator. This is essentially a one-stage system comprising thefollowing components:

a) An oxidation reactor (1) filled with an oxidation catalyst or with anadsorbing dispersion containing said oxidation catalyst and configured:

-   -   to receive a mixture of oxygen-enriched air streamed from an        oxygen concentrator and a flue gas containing NO_(x) and SO_(x);    -   to adsorb said gases on the particles of the oxidation catalyst;    -   to carry out catalytic oxidation of said NO_(x) and SO_(x) to        yield oxidised NO_(x) and SO_(x), and    -   to perform wet-scrubbing of said oxidised NO_(x) and SO_(x),        thereby yielding nitric and sulphuric acids;

b) A vessel containing gas or liquid ammonia, connected to the oxidationreactor (1) or to a separating and reactor-controlling unit (2), saidvessel having an inlet configured to stream said ammonia into theoxidation reactor (1) or into the separating and reactor-controllingunit (2) for reacting with the obtained nitric and sulphuric acids andto yield ammonium nitrate and ammonium sulphate fertilisers, therebywet-scrubbing NO_(x) and SO_(x) from the flue gas and producing saidfertilisers; and

c) The separating and reactor-controlling unit (2) connected to theoxidation reactor (1) and designed to separate the obtained products(fertilisers) and liquids, and to control said catalytic oxidationreaction and wet-scrubbing of the gases in the reactor (1).

The oxygen concentrator, which is not shown in this figure, is capableof concentrating oxygen from ambient air by selectively removingnitrogen from the air, thereby producing the air stream enriched withthe atmospheric oxygen for oxidation of the NO_(x) and SO_(x) gases. Ingeneral, any air containing more than 21% is considered anoxygen-enriched air. Concentration of oxygen in the oxygen-enriched airstrongly varies with the choice of equipment used for enriching air withoxygen. This equipment is not a part of the present invention, it iscommercially available and is not essential for the problem to besolved. Dependent on the brand/model of the external air generator usedfor preparing the oxygen-enriched air, the level of oxygen in airsupplied to the system of the present invention can strongly vary.Moreover, this parameter is not related to the process of the presentinvention and is not controlled in the process.

As noted above, practically any commercially available oxygenconcentrator can be used in the system of the embodiment. Most of themare based on fractional distillation. However, cryogenic oxygendistillators or oxygen concentrators based on membrane separation ofoxygen, pressure swing adsorption and vacuum pressure swing adsorptioncan also be used to produce the air stream of enriched atmosphericoxygen from ambient air.

In the second configuration shown in FIG. 1b , the system for combinedcatalytic oxidation and removal of NO_(x) and SO_(x) from a flue gascomprises:

a) An oxidation reactor (1) filled with an oxidation catalyst or with anadsorbing dispersion containing said oxidation catalyst and configured:

-   -   to receive a mixture of ambient air and the flue gas containing        NO_(x) and SO_(x);    -   to adsorb said gases on the particles of the oxidation catalyst;    -   to carry out catalytic oxidation of said NO_(x) and SO_(x) to        yield oxidised NO_(x) and SO_(x), and    -   to perform wet-scrubbing of said oxidised NO_(x) and SO_(x),        thereby yielding nitric and sulphuric acids;

b) A vessel containing gas or liquid ammonia, connected to the oxidationreactor (1) or to a separating and reactor-controlling unit (2), saidvessel having an inlet configured to stream said ammonia into theoxidation reactor (1) or into the separating and reactor-controllingunit (2) for reacting with the obtained nitric and sulphuric acids andto yield ammonium nitrate and ammonium sulphate fertilisers, therebywet-scrubbing NO_(x) and SO_(x) from the flue gas and producing saidfertilisers;

c) The separating and reactor-controlling unit (2) connected to saidoxidation reactor (1) and designed to separate the obtained products(fertilisers) and liquids, and to control said catalytic oxidationreaction and wet-scrubbing of the gases in the reactor (1), and

d) An activation chamber (8) for activating the oxidation catalyst, saidactivation chamber (8) contains an activation reagent and is in fluidcommunication with the separating and reactor-controlling unit (2), fromwhich it receives the deactivated oxidation catalyst, and with theoxidation reactor (1), to which it feeds the oxidation catalyst afterits activation.

The activation reagent in the activation chamber (8), capable ofactivating the deactivated oxidation catalyst at low temperatures about30-90° C., is selected from any suitable strong oxidising reagent,non-limiting examples of which are hydrogen peroxide (H₂O₂),hydroperoxides, such as tert-butyl hydroperoxide, benzoyl peroxide andthe like.

The system of the present invention may contain two types of theoxidation catalyst for facilitating the oxidation reaction of NO_(x) andSO_(x). The first catalyst is used in the form of solid catalystparticles suspended in water, while the second catalyst is soluble inorganic solvent and used in the oil-water emulsion. The term “adsorbingdispersion” used herein below thus defines both the aqueous suspensionof the oxidation catalyst particles (suspended in water) and theoil-water emulsion of the oxidation catalyst (soluble in organicsolvent). Depending on the type of the oxidation catalyst used or theircombination, there are several possible system configurations, which aredescribed in the present application:

(A) One-stage configuration based on the oxidation catalyst particlessuspended in water;

(B) Two-stage configuration based on the oxidation catalyst particlessuspended in water;

(C) One-stage configuration based on the combination of two catalysts:the first oxidation catalyst suspended in water and the oil-wateremulsion of the second oxidation catalyst (soluble in organic solvent);and

(D) Two-stage configuration based on the combination of two catalysts:the first oxidation catalyst suspended in water and the oil-wateremulsion of the second oxidation catalyst (soluble in organic solvent).

The configurations (C) and (D) differ from the configurations (A) and(B), respectively, in the design of their separating andreactor-controlling unit (2), which should be modified for separatingorganic and aqueous phases.

The above configurations may also contain different types of theoxidation reactor (1), which are selected from a bubble column, packedbed and spray tower. FIGS. 2a-2b show the one-stage system of thepresent invention operating either on oxygen-enriched air streamed froman oxygen concentrator (as in FIG. 2a ) or on atmospheric (ambient) airstreamed together with flue gas (as in FIG. 2b ), wherein the oxidationreactor (1) constitutes a spray tower. The spray tower can spray theadsorbing dispersion containing the oxidation catalyst. The oxidationcatalyst, in this case, may be either the solid catalyst suspended inwater, the catalyst soluble in organic solvent and emulsified in water,or combination thereof. The spray tower is a type of a wet scrubber usedto achieve mass and heat transfer between a continuous gas phase and adispersed liquid phase. The spray tower may consist essentially of anempty cylindrical vessel made of steel or plastic and nozzles that spraythe liquid into this vessel. The spray means may include one or morespray nozzles arrayed within the spray tower along the flue gas flowpath. The spray nozzles may be equipped with a demister (3) for mistremoval. In addition, a bottom tray (5) is used for forming a uniformgas flow in the tower cross section.

The obtained oxidation catalyst particles may be stabilised by thenegative charges of about 260 —Si—O— surface groups per particle withsodium ions Na⁺ as the counter ions. The pH of the obtained suspensionmay be adjusted with a basic solution, for example, sodium hydroxidesolution, to pH 8 or higher in order to hydrolyse cobalt on the silicaparticles (so called, “pH-jump”). The hydrolysis is carried out undervigorous stirring at room temperature. A Y-mixer with 20 mL/s flow ratemay be employed to provide uniform conditions for cobalt hydrolysis andadsorption on silica. The resultant suspension has a blue colour.

The size of the silica particles selected for the preparation of thecatalyst of the present invention is dependent on the size of sootparticles produced when the flue gases are emitted. Since the soot sizeranges between 5 to 600 nm, the smaller or larger silica particles areneeded. Based on extensive experimentation, the present inventors foundthat the large silica particles (more than 600 nm) constituting theoxidation catalyst of the present invention provide relatively lowcatalytic activity. The strongest catalytic activity was achieved usingCab-O-Sil M5 silica powder having 200-300 nm long chains of 10 nmspheres (supplier:http://www.cabotcorp.com/solutions/products-plus/fumed-metal-oxides/hydrophilic).The oxidation catalyst based on these silica particles allowed to remove88% NO_(x) from synthetic air-NO_(x) mixture.

Another silica used in the experiments performed by the presentinventors comprised 10-nm silica spheres, which is commerciallyavailable as 30% silica in water from Alfa Aesar (supplied by:https://www.alfa.com/en/catalog/043111/), in a form of a colloidaldispersion. This silica is on the lower edge of the soot's size andhence, active too.

As noted above, the catalyst using these types of silica was prepared bymixing cobalt sulphate (CoSO₄) with the silica followed by addition ofsodium hydroxide until the obtained suspension reaches pH 10 andmaintains this pH at a constant value. At such high pH, the formedcobalt hydroxide is not dissolved in water, which prevents leaching ofthe metal catalyst from the silica particles into water, therebypreserving its catalytic activity.

FIGS. 3a-3h show the SEM images of the obtained silica particles coatedwith cobalt hydrous oxide. The obtained Co(OH)₂/SiO₂ catalyst is a veryhigh surface-area, heterogeneous, yet suspendable in water catalyst thatdemonstrates both high selectivity and catalytic activity in theoxidation of the NO_(x) and SO_(x) gases by oxygen. The catalyst alsoshows high stability as no deactivation or precipitation of cobalt isobserved upon multiple cycling of cobalt ions through their higheroxidation state that must be involved in the oxidation process.According to the EDS analysis of cobalt atoms, the relative weight ofcobalt is 17.6% (σ=0.8) and the weight of oxygen is 82.4% (σ=0.8) in thecatalyst. As seen from these SEM images, the particles are composed ofessentially spheres, cobalt coats the particles and is not floating insolution, since there are no visible areas which contain only cobalt,and in the areas without silica, there is no visible cobalt.

Thus, the oxidation catalyst provides a system which does notnecessitate the utilisation of the expensive O₃ and H₂O₂ that areusually used. The Co(II) oxidation state changes to Co(III) after itsoxidation with oxygen. This oxidised cobalt is capable of oxidising NOand SO₂ in the flue gas, thereby being reduced back to Co(II). Theoxidation column is therefore filled with an aqueous suspension ofcobalt oxide/hydroxide particles supported on silica gel particles,thereby catalysing the oxidation of NO_(x) and SO_(x). When NO or SO₂,which is contained in the flue gas, is being absorbed on these catalystparticles, the irreversible oxidation reaction occurs at a finite buthigh speed according to the following equations:

The above oxidation reactions are considered to be globally of secondorder with respect to the reactants. Quite obviously an increase in theoxygen concentration (enrichment of the air stream with oxygen) enhancesthe rate of these oxidation reactions.

As shown in FIGS. 2a-2b , either the oxygen-enriched air stream and theflue gas, or the flue gas with ambient air enter the spray tower fromthe bottom and flow counter current to the adsorbing dispersion, whichis introduced at the top of the spray tower, sprayed downward the towerand adsorbs the oxidised NO_(x) and SO_(x) gases. The spray tower isoften packed with some inert solids, such as ceramic beads, in order topromote better contact between the two streams (oxygen and flue gas).

Separation of the oxidised NO_(x) and SO_(x), for instance, NO₂ and SO₃,from the reactant NO and SO₂ contained in the flue gas is achievedsimply because of the solubility of the former in water. The pH of theresulted suspension decreases due to the formation of nitric andsulfuric acids via the following reactions:

2NO₂+H₂O→HNO₃+HNO₂

SO₃+H₂O→H₂SO₄

As mentioned in the background, NO₂ exists in equilibrium with thecolourless gas dinitrogen tetroxide (N₂O₄):2NO₂⇄N₂O₄. Also, therelatively unstable dinitrogen trioxide (N₂O₃) may be formed accordingto the equilibrium: NO+NO₂⇄N₂O₃. In the presence of water, NO and NO₂may also exist in the equilibrium with nitrous acid: NO+NO₂+H₂O⇄2HNO₂.The concentrations of the various nitrogen oxides species present duringthe entire adsorption-oxidation process (NO, NO₂, N₂O₃, N₂O₄) are notindependent though, which complicates the whole process. When absorbedinto water NO₂, N₂O₃ and N₂O₄ undergo relatively fast hydrolysis,thereby producing nitric acid (HNO₃) and nitrous acid (HNO₂), the latterbeing decomposed in nitrous oxide NO, which desorbs to the gas phaseaccording to the reaction: 3HNO₂⇄HNO₃+H₂O+2NO. The pH during the entireprocess must be maintained in the range of 4-7 with ammonium hydroxideinjection in order to keep the reaction going and not to create thealkaline solution, from which ammonia gas (NH₃) may evolve. Thesuspension colour may change from pink (acidic) to blue (alkaline)depending on the pH of the solution. The operating temperature is 60-70°C. due to the hot flue gas coming from the furnace.

The term “mother liquor” used herein below defines the liquid portion ofthe circulating adsorbing dispersion that contains almost no suspendedor dissolved oxidation catalyst or crystallisation product. It is eitherrecycled into the spray tower together with the oxidation catalystparticles in a form of the aqueous suspension, or sprayed from thenozzles on the dry oxidation catalyst particles floating in the spraytower, thereby forming the aqueous suspension directly inside thereactor. The mother liquor is also the liquid left over aftercrystallisation of the fertiliser products and collected by filteringoff the crystals.

As schematically shown in FIGS. 2a and 2b , the adsorbing dispersion isfiltered and recycled continuously in the system. The spray tower isconnected to a vessel (not shown in the figure) containing either gas orliquid ammonia, which is streamed into a phase separator inside theseparating and reactor-controlling unit (2) in order to react with theoxidised NO_(x) and SO_(x) species, thereby producing fertilisers(ammonium nitrate and ammonium sulphate) according to the followingequation: HNO₃ (aq)+H₂SO₄ (aq)+3NH₄OH→NH₄NO₃ (s)+(NH₄)₂SO₄ (s)+3H₂O.

When the ammonia-containing vessel contains ammonia in a gas phase, thevessel is pressurised (the pressure vessel). Alternatively, ammoniawhich is streamed into the system, may be in a liquid form (in itsaqueous solution), and then the vessel is a regular container forliquids with a pump pumping the aqueous ammonia solution into the phaseof the separator and reactor control unit (2). The obtained solidproducts (fertilisers) are then separated from the liquids andcollected. The system of the present invention may further comprisesensors for measuring and controlling pH and temperature of the liquid.The mother liquor may be further recycled by transferring it for feedinga new portion of the suspension in the spray tower. The adsorbingdispersion sprayed in the spray tower may contain the filtered aqueoussolution that is recycled from the dry oxidation chamber and containsthe dissolved NH₄NO₃ and (NH₄)₂SO₄ fertiliser products and ammoniumhydroxide.

Another oxidation catalyst that may be used in the system of the presentinvention is elemental sulphur capable of catalysing the NO_(x) andSO_(x) oxidation reaction. In that case, the adsorbing dispersion mustcontain an oil-water emulsion of sulphur, wherein the organic phase ofthe emulsion comprises elemental sulphur in saturated heavy mineral oil.Thus, in a particular embodiment, the sprayed adsorbing dispersion mayfurther contain an organic phase comprising elemental sulphur insaturated heavy mineral oil, which acts as an additional oxidationcatalyst in the NO_(x) and SO_(x) oxidation process.

In other words, when the adsorbing dispersion is an oil-water emulsionof the oxidation catalyst, the mother liquor may be an emulsioncontaining a filtered aqueous solution recycled from the spray tower andan organic phase comprising elemental sulphur in saturated heavy mineraloil. Said sulphur is capable of catalysing the oxidation reaction of theNO_(x) and SO_(x) species.

The sulphur catalyst for the above reaction is prepared by addition ofthe elemental sulphur into heavy mineral oil at a temperature higherthan the melting point of sulphur (119° C.). Dissolution of sulphur inthe heavy mineral oil is therefore carried out at elevated temperaturesin the range 120-160° C. and results in partial formation of the S—Obonds (sulfoxides), which are efficient catalysts in oxidation of(nitrite) NO₂ ⁻ and (sulphite) SO₃ ²⁻ into nitrate (NO₃ ⁻) and sulphate(SO₄ ²⁻), respectively. The formed water-soluble NO₃ ⁻ and SO₄ ²⁻ ionsthen undergo a phase transfer diffusing from the organic phase to theaqueous phase (the latter is in contact with the droplets of the mineraloil catalyst) in the spray tower.

Thus, the obtained sulphur oil solution containing the mineral oil, inwhich the elemental sulphur is dissolved up to saturation, creates anemulsion when mixed with the aqueous solution streamed from theoxidation column. The resultant oil/water emulsion is used along withthe aqueous suspension of the oxidation catalyst particles. The organicphase may further comprise various activators that may be added to theoil sulphur solution to increase the solubility of the elementalsulphur, such as diphenyl ether, dichlorobenzene, decabromo-diphenylether or Disperbyk®-108. These activators are added to the sulphur oilsolution to increase the solubility of the elemental sulphur. Thesulfoxide appears to be an active species which plays a major role inthe oxidation processes. In addition, the catalyst oil phase which is abad solvent for ionic species may play a role in the efficient migrationof the ammonium salts (nitrate and sulphate), their separation from theproducts upon saturation in the aqueous phase, followed by precipitationas solid salts which are used as fertilisers.

Reference is now made to FIGS. 4a and 4c schematically showing theseparating and reactor-controlling unit (2) for the two systemconfigurations shown in FIGS. 1a-1b and 2a-2b of the present invention,respectively. The structure, configuration and components of theseparating and reactor-controlling unit (2) depend on whether theadsorbing dispersion constitutes only an aqueous suspension of theoxidation catalyst particles, or the suspension also contains anoil-water emulsion of the emulsified oxidation catalyst. FIGS. 4b and 4dschematically show the separating and reactor-controlling unit (2) forthe two system configurations shown in FIGS. 1a-1b and 2a-2b of thepresent invention, respectively, but where the adsorbing dispersioncontains the oil-water emulsion of the catalyst. The stream of theoil-water emulsion obtained in the wet scrubbing process is transferredto the oil-water separator inside the separating and reactor-controllingunit (2) for separating gross amounts of oil from water and suspendedsolids. Any available oil-water separator may be used for this purpose,for example, an API separator, gravity plate separator, centrifugalseparator, hydrocyclone separator, electro-chemical separator ordownhole separator. The separated oil is then returned to the spraytower, while the aqueous phase containing dissolved NH₄NO₃ and (NH₄)₂SO₄products is transferred to a crystallisation vessel (not shown in thefigure) for further crystallisation and precipitation of NH₄NO₃ and(NH₄)₂SO₄ from the aqueous solution. The mother liquor is then recycledback into the spray tower.

In general, the separating and reactor-controlling unit (2) is used forhandling liquid streams transferred from the oxidation reactor (1),including a full stream or portion of it, which is called “bleedstream”. In the unit (2), the streamed adsorbing dispersion with thedissolved and oxidised NO_(x) and SO_(x) is allowed to contact with theinjected stream of ammonia to produce the ammonium nitrate and ammoniumsulphate fertiliser products. This reaction is carried out inside theunit (2), followed by separation of the products from the circulatingstream. The unit (2) therefore comprises at least one of the followingprocessing sub-units: a phase separator, crossflow separator,mixer-settler, decanter, tricanter or filter. When the adsorbingdispersion contains an oil-water emulsion, the unit (2) comprises anoil-water phase separator configured to receive said oil-water emulsionfrom the adsorption and oxidation reactor and separate the oil (organicphase) from water (aqueous phase) containing salts and suspended solidcatalyst particles. This separation can be done using gravity orcentrifugal separators.

The separating and reactor-controlling unit (2) may further comprisesensors for measuring and controlling temperature and pH of theprocessed liquids. The flow, temperature and pH feedback control isperformed by measuring the present values and relating them to thereference values using various actuators, such as an electric heater(for temperature control), ammonia dosing pump (for pH control), andcontrolled main pump (for flow control).

Separation of the suspended catalyst particles from the aqueous solutionor mother liquor can be carried out by membrane filtration, for exampleusing a crossflow filtration sub-unit. For the filtration process andfor the inlet stream, additional pressure pumps may be needed. Theaqueous solution containing the dissolved ammonium nitrate and ammoniumsulphate products is removed from the system, while the mother liquor,suspended catalyst particles, the organic phase and added water aremixed and steamed back to the oxidation reactor (1) at the bottom orthrough the nozzles. This way, the adsorbing dispersion containing thecatalyst is allowed to circulate inside the system, while the fertiliserproducts are continuously removed from the system.

In a further embodiment, the combined system of the present inventioneither comprises a separate oxidation chamber connected to the oxygenconcentrator or receives atmospheric air, and consequently designed toreceive either a mixture of oxygen-enriched air and flue gas, or fluegas with ambient air, respectively. FIGS. 5a and 5b show the operationaldiagram of the industrial two-stage system of the present invention inthe two configurations defined above, having a separate oxidationchamber (4) containing the oxidation catalyst. This oxidation chamber(4) is configured to either receive an oxygen-enriched air stream andflue gas stream containing NO_(x) and SO_(x) (as shown in FIG. 5a ) orto receive the flue gas with ambient air (see FIG. 5b ), and to carryout the oxidation reaction of these gases with said oxidation catalyst.The wet scrubber (1) in this case may be essentially the same oxidationreactor (1) shown in FIGS. 1a and 1b for the one-stage system of thepresent invention.

The wet scrubber (1) contains an adsorbing dispersion, receives thestreams of the air and flue gas containing the oxidised NO_(x) andSO_(x), adsorbs the streamed gases onto the catalyst particles of theadsorbing dispersion and then carries out the wet scrubbing of saidgases. The adsorbing dispersion in this case is recycled in the systemthe same way as explained above for the one-stage configuration andtherefore contains the oxidation catalyst capable of completing thesimultaneous oxidation of NO_(x) and SO_(x) partially pre-oxidised inthe oxidation chamber (4), if necessary.

FIGS. 6a and 6b schematically show the two-stage system of the presentembodiment in the two configurations defined above, respectively,wherein the wet scrubber (1) is a spray tower that is capable ofspraying the adsorbing dispersion. In the first configuration, shown inFIG. 6a , the oxidation chamber (4) is connected to the oxygenconcentrator (not shown here) to receive the air stream enriched withatmospheric oxygen and the stream of a flue gas containing NO_(x) andSO_(x). In the second configuration, shown in FIG. 6b , the oxidationchamber (4) receives the flue gas with ambient air.

The oxidation chamber (4) shown in FIGS. 6a-6b may be either dry, packedwith inert solids, such as ceramic beads, promoting a better contactbetween said air stream and said flue gas stream, or wet with a liquidcirculating inside. Consequently, the oxidation chamber (4) is filledwith either dry catalyst particles or wet catalyst particles and capableof carrying out the catalytic oxidation of the flue gases by thesupplied air. The oxidation chamber (4) may then be refilled with freshwater or with water recycled from the mother liquor left afterprecipitation or crystallisation of the fertiliser products.

As shown in FIGS. 6a-6b , the gas stream containing oxidised NO_(x) andSO_(x) enters from the dry oxidation chamber (4) at the bottom of thespray tower and moves (flows) upward counter current to the adsorbingdispersion, which is sprayed downward from one or more nozzles. Thus,the two-stage system shown in FIGS. 5a-5b and 6a-6b may actually betransformed into a one-stage system, when the oxidation chamber (4) andspray tower (1) are combined together or the oxidation chamber isincorporated into the spray tower.

FIGS. 7 and 8 schematically show the one- and two-stage laboratorysystems of the present invention corresponding to those shown in FIGS.2a and 6a , respectively, and described above. Further, FIG. 9schematically shows the operational diagram of the one-stage laboratorysystem fed with synthetic gases, while FIG. 10 schematically shows theoperational diagram of the one-stage laboratory system fed with the fuelgases from a diesel engine.

In another embodiment, a method for catalytic oxidation and removal ofnitrogen oxides (NO_(x)) and sulphur oxides (SO_(x)) simultaneously froma flue gas containing said oxides, comprises:

I. Catalytic oxidation of the NO_(x) and SO_(x) contained in the fluegas with oxygen, wherein said catalytic oxidation yields the oxidisedNO_(x) and SO_(x), said catalytic oxidation is carried out in theoxidation reactor (1) of the system of claim 1; and

II. Wet-scrubbing of the oxidised NO_(x) and SO_(x) with an adsorbingdispersion comprising a solid oxidation catalyst suspended in water, anoxidation catalyst soluble in organic solvent and emulsified in water,or combination thereof, thereby removing the NO_(x) and SO_(x) from theflue gas and yielding nitric and sulphuric acids dissolved in water.

The method of the present invention may further comprise at least one ofthe following steps:

III. Simultaneously removing the nitric and sulfuric acids from water bycontacting the adsorbing dispersion with ammonia to produce ammoniumnitrate and ammonium sulphate;

IV. Separation, crystallisation and collection of the ammonium nitrateand ammonium sulphate fertiliser products; and

V. Recycling water from mother liquor, said mother liquor is left afterprecipitation of said ammonium nitrate and ammonium sulphate.

In a specific embodiment, the above process is carried out at therelatively low temperature of about 30-90° C. and pH 4-7. This pH ismaintained with ammonium hydroxide injection in order to keep thereaction going and not to create the alkaline solution, from whichammonia gas (NH₃) may evolve.

The combined system of the embodiment is thus built to replace theexisting bulky and expensive wet scrubbing systems operating at arelatively high temperature (more than 300° C.) with much smaller,simpler and cheaper combination systems operating at about 30-90° C.This heat originates from thermodynamic equilibrium in the process, andno additional heat is required and applied.

The system of the present invention is available in two configurationsas described above (with and without enriching air with oxygen). Anotherimportant feature of the present invention is that the process foroxidation of NO_(x) and SO_(x) in flue gases is a simple, economicprocess providing a general method for manufacturing fertilisers, suchas ammonium nitrate and ammonium sulphate, by ammonia injection to theoxidised NO_(x) and SO_(x) gases.

The combined system of the embodiment and the process carried outtherein allow the replacement of the present expensive clean fossilfuels (purified according to environmental regulations and used in powerproduction) with relatively much cheaper nitrogen and sulphurcontaminated fossil fuels, which may not be used with existing systemsbecause of their hazardous effect on the environment. Also, thepossibility of combined treatment for different types of pollutants atthe same facility and at relatively low temperature is of greatoperational and economic advantage.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

1. A combined system for catalytic oxidation and wet-scrubbing ofsimultaneously both nitrogen oxides (NO_(x)) and sulphur oxides (SO_(x))from a flue gas, and for manufacturing fertilisers comprising: a) Anoxidation reactor (1) filled with an oxidation catalyst or with anadsorbing dispersion containing said oxidation catalyst and designed: toreceive either (i) a mixture of oxygen-enriched air streamed from anoxygen concentrator and a flue gas containing NO_(x) and SO_(x), or (ii)a mixture of ambient air and the flue gas containing NO_(x) and SO_(x);to adsorb said gases on the particles of the oxidation catalyst; tocarry out catalytic oxidation of said NO_(x) and SO_(x) to yieldoxidised NO_(x) and SO_(x), and to perform wet-scrubbing of saidoxidised NO_(x) and SO_(x), thereby yielding nitric and sulphuric acids;b) A vessel containing gas or liquid ammonia, connected to the oxidationreactor (1) or to a separating and reactor-controlling unit (2), saidvessel having an inlet configured to stream said ammonia into theoxidation reactor (1) or into the separating and reactor-controllingunit (2) for reacting with the obtained nitric and sulphuric acids andto yield ammonium nitrate and ammonium sulphate fertilisers, therebywet-scrubbing NO_(x) and SO_(x) from the flue gas and producing saidfertilisers; and c) The separating and reactor-controlling unit (2)connected to said oxidation reactor (1) and designed to separate theobtained products (fertilisers) and liquids, and to control saidcatalytic oxidation reaction and wet-scrubbing of the gases in thereactor (1), wherein if the oxidation reactor (1) is configured toreceive the mixture of ambient air and the flue gas containing NO_(x)and SO_(x), said system further comprises an activation chamber (8) foractivating the oxidation catalyst, said activation chamber (8) containsan activation reagent and is in fluid communication with the separatingand reactor-controlling unit (2), from which it receives the deactivatedoxidation catalyst, and with the oxidation reactor (1), to which itfeeds the oxidation catalyst after its activation.
 2. The system ofclaim 1, comprising: a) An oxidation reactor (1) filled with anoxidation catalyst or with an adsorbing dispersion containing saidoxidation catalyst and configured: to receive a mixture ofoxygen-enriched air streamed from an oxygen concentrator and a flue gascontaining NO_(x) and SO_(x); to adsorb said gases on the particles ofthe oxidation catalyst; to carry out catalytic oxidation of said NO_(x)and SO_(x) to yield oxidised NO_(x) and SO_(x), and to performwet-scrubbing of said oxidised NO_(x) and SO_(x), thereby yieldingnitric and sulphuric acids; b) A vessel containing gas or liquidammonia, connected to the oxidation reactor (1) or to a separating andreactor-controlling unit (2), said vessel having an inlet configured tostream said ammonia into the oxidation reactor (1) or into theseparating and reactor-controlling unit (2) for reacting with theobtained nitric and sulphuric acids and to yield ammonium nitrate andammonium sulphate fertilisers, thereby wet-scrubbing NO_(x) and SO_(x)from the flue gas and producing said fertilisers; and c) The separatingand reactor-controlling unit (2) connected to the oxidation reactor (1)and designed to separate the obtained products (fertilisers) andliquids, and to control said catalytic oxidation reaction andwet-scrubbing of the gases in the reactor (1).
 3. The system of claim 2,further comprising a separate oxidation chamber (4) connected to theoxygen concentrator and configured to receive the oxygen-enriched airstream from the oxygen concentrator and a stream of the flue gascontaining NO_(x) and SO_(x), said oxidation chamber (4) is filled withthe oxidation catalyst capable of catalysing oxidation of NO_(x) andSO_(x) in the flue gas by said oxygen, said oxidation chamber (4) iseither dry and packed with inert solids promoting a better contactbetween said oxygen stream and said flue gas stream, or wet andcontaining a liquid circulating inside.
 4. The system of claim 1,comprising: a) An oxidation reactor (1) filled with an oxidationcatalyst or with an adsorbing dispersion containing said oxidationcatalyst and configured: to receive a mixture of ambient air and theflue gas containing NO_(x) and SO_(x); to adsorb said gases on theparticles of the oxidation catalyst; to carry out catalytic oxidation ofsaid NO_(x) and SO_(x) to yield oxidised NO_(x) and SO_(x), and toperform wet-scrubbing of said oxidised NO_(x) and SO_(x), therebyyielding nitric and sulphuric acids; b) A vessel containing gas orliquid ammonia, connected to the oxidation reactor (1) or to aseparating and reactor-controlling unit (2), said vessel having an inletconfigured to stream said ammonia into the oxidation reactor (1) or intothe separating and reactor-controlling unit (2) for reacting with theobtained nitric and sulphuric acids and to yield ammonium nitrate andammonium sulphate fertilisers, thereby wet-scrubbing NO_(x) and SO_(x)from the flue gas and producing said fertilisers; c) The separating andreactor-controlling unit (2) connected to said oxidation reactor (1) anddesigned to separate the obtained products (fertilisers) and liquids,and to control said catalytic oxidation reaction and wet-scrubbing ofthe gases in the reactor (1), and d) An activation chamber (8) foractivating the oxidation catalyst, said activation chamber (8) containsan activation reagent and is in fluid communication with the separatingand reactor-controlling unit (2), from which it receives the deactivatedoxidation catalyst, and with the oxidation reactor (1), to which itfeeds the oxidation catalyst after its activation.
 5. The system ofclaim 4, further comprising a separate oxidation chamber (4) configuredto receive the flue gas containing NO_(x) and SO_(x) together withambient air, said oxidation chamber (4) is filled with the oxidationcatalyst capable of catalysing oxidation of NO_(x) and SO_(x) in theflue gas, and said oxidation chamber (4) is either dry and packed withinert solids promoting a better contact between said oxygen stream andsaid flue gas stream, or wet and containing a liquid circulating inside.6. The system of claim 1, wherein said oxidation reactor (1) is eitherdry and packed with inert solids promoting a better contact between saidoxygen stream and said flue gas stream, or wet and containing a liquidcirculating inside.
 7. The system of claim 1, wherein said oxidationreactor (1) is selected from a bubble column, packed bed and spraytower.
 8. The system of claim 8, wherein said oxidation reactor (1) is aspray tower equipped with spray means capable of spraying into saidspray tower either (i) water or mother liquor onto the dry oxidationcatalyst particles, thereby forming floating drops of the adsorbingdispersion directly inside the spray tower, or (ii) the adsorbingdispersion prepared in advance and comprising the oxidation catalystparticles.
 9. The system of claim 1, wherein said adsorbing dispersionis an aqueous suspension of the oxidation catalyst particles.
 10. Thesystem of claim 9, wherein said oxidation catalyst comprises the mixtureof an aqueous solution of a metal salt precursor with silica gelparticles and used for catalysing the oxidation reaction of NO_(x) andSO_(x) in the flue gas.
 11. The system of claim 10, wherein the metalsalt precursor is a water-soluble inorganic salt of a transition metalselected from cobalt (Co), nickel (Ni), manganese (Mn), iron (Fe),copper (Cu) and chromium (Cr).
 12. The system of claim 10, wherein theoxidation catalyst comprises an aqueous suspension of cobaltoxide/hydroxide particles supported on silica gel particles.
 13. Thesystem of claim 1, wherein said separating and reactor-controlling unit(2) comprises at least one of the following processing units: a phaseseparator, crossflow separator, mixer-settler, decanter, tricanter orfilter.
 14. The system of claim 13, further comprising a crystallisationvessel connected to the processing unit and configured to receive fromsaid processing unit an aqueous solution containing the dissolvedammonium nitrate and ammonium sulphate products, said crystallisationvessel is capable of crystallising and precipitating the ammoniumnitrate and ammonium sulphate from the aqueous solution.
 15. The systemof claim 1, wherein said adsorbing dispersion is an oil-water emulsion,and said separating and reactor-controlling unit (2) comprises anoil-water phase separator configured to receive said oil-water emulsionfrom the oxidation reactor (1) and to separate gross amounts of oil(organic phase) from water (aqueous phase).
 16. The system of claim 15,wherein the oil-water emulsion comprises a filtered aqueous solution andan organic phase, said organic phase comprises elemental sulphur insaturated heavy mineral oil, said sulphur is capable of catalysing theoxidation reaction of NO_(x) and SO_(x) in the flue gas.
 17. The systemof claim 16, wherein said organic phase further comprises activatorsadded to the sulphur oil solution to increase solubility of theelemental sulphur.
 18. A method for catalytic oxidation and simultaneousremoval of nitrogen oxides (NO_(x)) and sulphur oxides (SO_(x)) from aflue gas containing said oxides, comprising: I. Catalytic oxidation ofthe NO_(x) and SO_(x) contained in the flue gas with oxygen, whereinsaid catalytic oxidation yields the oxidised NO_(x) and SO_(x), saidcatalytic oxidation is carried out in the oxidation reactor (1) of thesystem of claim 1; and II. Wet-scrubbing of the oxidised NO_(x) andSO_(x) with an adsorbing dispersion comprising a solid oxidationcatalyst suspended in water, an oxidation catalyst soluble in organicsolvent and emulsified in water, or combination thereof, therebyremoving the NO_(x) and SO_(x) from the flue gas and yielding nitric andsulphuric acids dissolved in water.
 19. The method of claim 18, furthercomprising at least one of the steps selected from: III. Simultaneouslyremoving the nitric and sulfuric acids from water by contacting theadsorbing dispersion with ammonia to produce ammonium nitrate andammonium sulphate; IV. Separation, crystallisation and collection of theammonium nitrate and ammonium sulphate fertiliser products; and V.Recycling water from mother liquor, said mother liquor is left afterprecipitation of said ammonium nitrate and ammonium sulphate.
 20. Themethod of claim 18, wherein said method is carried out at thetemperature of about 30-90° C. and pH 4-7.